In data communication or data storage it is common practice to transmit or store data with redundancy in a coded manner in order to improve reliability of being able to recreate the original message. The process is usually referred to as channel coding and the recovery process as channel decoding. We will refer to such a message as a code word even though in the following it does not strictly have to be encoded.
In communication systems, such as, e.g., the Long Term Evolution (LTE) system standardized by the Third Generation Partnership Project (3GPP), it is also common to combine several transmissions relating to the same code word in different transmission time intervals (TTIs) if needed to adaptively increase the level of redundancy to the transmission conditions. This can for instance be done by repeating a shorter coded or uncoded message one or several times. An alternative is to transmit a part of a code word containing sufficient information for correct decoding under favorable conditions, in a first transmission attempt. If not received and decoded correctly, additional parts of the code word can be transmitted in subsequent attempts after which the received parts of the code word can be recombined on the receiver side, creating a redundancy which is incremental for each retransmission. This can then help in making sure that sufficient but not more resources than necessary are used for transmission of each message. For brevity we will refer to subsequent transmissions of the same code words as retransmissions even though it may not be the whole code word that is being retransmitted. The information bits carried by a code word will be referred to as a transport block (TB).
In order for transmission of subsequent code words not to be delayed while waiting for previous messages being decoded and potentially being (partly) retransmitted, a set of buffers containing the data of different code words exist in parallel. This way other buffers can be read for (re)transmission while waiting for the previous transmission of the same transport block to be decoded and for messages of correct/incorrect reception to be received at the transmitter side (acknowledged (ACK) or not acknowledged (NACK) messages). These buffers are usually referred to as Hybrid Automatic Repeat reQuest (Hybrid ARQ or HARQ) buffers and the process controlling each of them is referred to as a HARQ process.
HARQ re-transmissions are handled by the Medium Access Control (MAC) layer which is part of Layer 2 (L2) in the LTE protocol architecture. HARQ feedback, i.e. ACK or NACK indication, is signaled to the MAC layer from the physical layer, also referred to as Layer 1. Layer 2 uses this information in its data transfer process to either make a retransmission or a new transmission.
Multi-antenna techniques can significantly increase the data rates and/or reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas. This results in a multiple-input multiple-output (MIMO) communication channel and such systems and/or related techniques are commonly referred to as MIMO techniques.
One MIMO technique is Spatial Multiplexing (SM), or Single User MIMO (SU-MIMO), where one or several transport blocks relating to one specific user are simultaneously mapped (usually linearly) to one or several layers of data which in turn are mapped, potentially via channel adaptive precoders (also often linear precoders), to the different transmit antenna ports. Currently for LTE, one or two codewords, corresponding to one or two transport blocks, are mapped to the one or several layers of data. This way the spatial properties of the MIMO channel can, under favorable conditions, be exploited to transmit more data simultaneously relating to the same user, increasing the user data throughput. There may also be additional intermediate processing steps for various reasons.
In LTE Release 10 (Rel. 10), the uplink (UL), which is the communication link from user equipment to base station, or evolved NodeB (eNB) in LTE terminology, is being extended from supporting single-input single-output (SISO) to also support UL-Spatial Multiplexing (UL-SM).
As in previous releases (Rel-8 and Rel-9), an UL transmission is triggered via an uplink transmission grant transmitted on the Physical Downlink Control Channel (PDCCH). Retransmissions however can either be triggered by a full grant transmitted on the PDCCH or, if no PDCCH grant is found for the corresponding transport block, by a non-acknowledgement indication, NACK, on the Physical HARQ Indicator Channel (PHICH) indicating that the decoding of previous transmission attempt of the corresponding code word failed. The former retransmission type is usually referred to as an adaptive retransmission as the PDCCH grant format allows for specifying a new transport format (e.g., modulation constellation and code rate). The latter type of retransmission is consequently referred to as a non-adaptive retransmission as the PHICH carries only the indication of ACK or NACK of the previous transmission and gives no other signaling possibility to order the UE to use a new transport format.
In LTE, UL Synchronous HARQ is employed, which means that there is a fixed timing relation between transmission and retransmission, hence there is a direct mapping from TTI to HARQ process identity (ID) and this information is not needed in the UL grant. When there are limited PDCCH resources, the base station can therefore grant a UE an UL retransmission by a PHICH NACK alone which then has a reduced involvement of Layer 2, L2, resources compared to a grant received on the PDCCH. A drawback is that no new information on transport format can then be conveyed to the UE such as link adaptation or frequency selective rescheduling. The reliability of the PHICH channel is also lower than that of the PDCCH grant.
In the LTE downlink, DL, however, Asynchronous HARQ is employed, and an explicit PDCCH assignment is needed to point out that a DL (re)transmission is related to a specific DL HARQ process. For DL spatial multiplexing there is therefore always an assignment for retransmission of any code word.
This means that for LTE when DL spatial multiplexing is configured, the physical layer, or Layer 1, L1, of the UE reads the PDCCH for a DL assignment and when a downlink assignment is detected, it will furthermore detect if the assignment is valid for one or two transport blocks. This means that if the PDCCH signaling indicates no assignment for one of the transport blocks, for example TB1, the UE will not read the Physical Downlink Shared Channel (PDSCH) for data for this transport block. For TB2 it will however read the PDSCH according to the PDCCH to detect the corresponding code word that represents data. The data is then forwarded to L2, or the Medium Access Control (MAC) layer, and the appropriate HARQ process for decoding.
In the case where UL-SM is configured, the UE may, for each TTI, be assigned an UL grant that is valid for one or two TBs. It is assumed that L1 will detect if the grant is valid for one or two TB(s) based on the explicit PDCCH signaling, similar to how it is done for DL spatial multiplexing. The reason for disabling a transport block may be that the UE buffer might be empty, or the MIMO channel may not be sufficiently rich to be able to convey multiple data layers.
It should be noted that for spatial multiplexing the notion of a single grant valid for one or two transport blocks is practically equivalent to that of one or two grants valid for one transport block each. The difference is only semantic, and is henceforth used interchangeably.
The current 3GPP MAC Layer specification procedure for UL data transfer is able to handle only one UL grant (or lack of UL grant) per TTI, hence some complication can be expected when one transport block is assigned an UL grant and the other is not. Since these two branches are mutually exclusive in the current specifications, it would be more straightforward to handle each transport block separately, i.e., to assume that L2 receives individual grants per transport block and that each transport block is associated with a separate HARQ process. That way, the grant reception procedure should be iterated once for each grant associated with a certain TTI.
Assuming that the procedure is executed separately for each transport block, the different branches could be executed for the different cases of one transport block, e.g., TB1, having no UL grant and the other transport block, e.g., TB2, having an UL grant.